For best results, we recommend modifying our standard button code to replace our standard button image with a custom button, rather than starting with a custom button and trying to link that to the cart. The top of this help page explains how to do that:

Since you are using text within SPAN tag for your button you'd have to format the text for the View Cart button differently so that it lines up better with the Add to Cart button, but I don't know that it would display the same in all browsers. If you want to get perfectly matching buttons I'd recommend using images.